I. Field
The subject specification relates generally to wireless communications and, more particularly, to handover mechanism(s) for handover of a mobile terminal in a wireless communication system.
II. Background
Conventional technologies utilized for transmitting information within a mobile communication network (e.g., a cell phone network) include frequency, time and code division based techniques. In general, with frequency division based techniques calls are split based on a frequency access method, wherein respective calls are placed on a separate frequency. With time division based techniques, respective calls are assigned a certain portion of time on a designated frequency. With code division based techniques respective calls are associated with unique codes and spread over available frequencies. Respective technologies can accommodate multiple accesses by one or more users.
More particularly, frequency division based techniques typically separate the spectrum into distinct channels by splitting it into uniform chunks of bandwidth, for example, division of the frequency band allocated for wireless cellular telephone communication can be split into 30 channels, each of which can carry a voice conversation or, with digital service, carry digital data. Each channel can be assigned to only one user at a time. One commonly utilized variant is an orthogonal frequency division technique that effectively partitions the overall system bandwidth into multiple orthogonal subbands. These subbands are also referred to as tones, carriers, subcarriers, bins, and frequency channels. Each subband is associated with a subcarrier that can be modulated with data. With time division based techniques, a band is split time-wise into sequential time slices or time slots. Each user of a channel is provided with a time slice for transmitting and receiving information in a round-robin manner. For example, at any given time t, a user is provided access to the channel for a short burst. Then, access switches to another user who is provided with a short burst of time for transmitting and receiving information. The cycle of “taking turns” continues, and eventually each user is provided with multiple transmission and reception bursts.
Code division based techniques typically transmit data over a number of frequencies available at any tune in a range. In general, data is digitized and spread over available bandwidth, wherein multiple users can be overlaid on the channel and respective users can be assigned a unique sequence code. Users can transmit in the same wide-band chunk of spectrum, wherein each user's signal is spread over the entire bandwidth by its respective unique spreading code. This technique can provide for sharing, wherein one or more users can concurrently transmit and receive. Such sharing can be achieved through spread spectrum digital modulation, wherein a user's stream of bits is encoded and spread across a very wide channel in a pseudo-random fashion. The receiver is designed to recognize the associated unique sequence code and undo the randomization in order to collect the bits for a particular user in a coherent manner.
A typical wireless communication network (e.g., employing frequency, time and code division techniques) includes one or more base stations that provide a coverage area and one or more mobile (e.g., wireless) terminals that can transmit and receive data within the coverage area. A typical base station can simultaneously transmit multiple data streams for broadcast, multicast, and/or unicast services, wherein a data stream is a stream of data that can be of independent reception interest to a mobile terminal. A mobile terminal within the coverage area of that base station can be interested in receiving one, more than one or all the data streams carried by the composite stream. Likewise, a mobile terminal can transmit data to the base station or another mobile terminal. Such communication between base station and mobile terminal or between mobile terminals can be degraded due to channel variations and/or interference power variations. For example, the aforementioned variations can affect base station scheduling, power control and/or rate prediction for one or more mobile terminals.
In the foregoing wireless communication systems, handover decisions are typically based on downlink (DL) channel quality metrics, for substantially any suitable metric, among the target base station and the access terminal to be handed-off. Such conventional approach to handoff resolution fails to incorporate uplink (UL) channel quality indications of the target cell. Yet, UL and DL in a typical wireless communication design may have substantially disparate characteristics, and therefore present an imbalance between a quality of UL and DL transmission and reception—generally referred to as link imbalance. In addition, disparate propagation environments for UL and DL signal may lead to further disparities in UL and DL channel quality. Therefore, handover decisions that rely only on a set of DL quality indications of a target base station may be inadequate, especially in cases where link quality imbalance is such that DL channel condition may be above a threshold for handoff, but UL channel conditions may be below such a threshold. There is therefore a need in the art for handover mechanism(s) that relies on both DL and UL channel quality.